Sealed resistor



United st tes Patent This invention relates to sealed resistors and particularly to resistors having solder seals at the ends of ceramic shells.

In the past it has been found that when a ceramic resistor is sealed into a ceramic tube, the surfaces at the ends of which have been metallized, by closing the ends with plugs of molten solder, the metallized coating may adhere to the solder so firmly as to break away from the outer ceramic shell under severe temperature cycling such as is now required by military specifications. Alternatively, if the metallized coating does not break away from the shell, the entire shell may be cracked.

In the present invention a resistor is made by coating a ceramic rod with a suitable resistive material, suchas a metal film or a coating of carbonaceous material, and forcing a pair of metal caps over the ends of the rod to connect with the ends of the coating. The coated ceramic rod with the end caps attached to it is then inserted into a ceramic shell of ceramic material having substantially the same linear coefficient of thermal expansion as the rod. The term linear refers to the low-temperature region of the coeificient of thermal expansion characteristic of the material since the expansion of ceramic material is usually non-linear at high temperatures. The inner surface of the shell is metallized at its ends, and after the rod is properly positioned within the shell, molten solder is placed in the space at each end of the shell to adhere to the metallized band and to the caps, particularly the cylindrical parts thereof that extend longitudinally in the annular gap between the rod and the metallized bands.

In accordance with the present invention the metal caps are made of a material having a coefficient ofthermal expansion approximately equal to the linear coefiicient of thermal expansion of the ceramic material. This prevents the expansion of the cap under temperature cycling from pulling the inner end of the metallized band away from the ceramic shell and it also prevents the shell from breaking. This maintains the protective enclosure intact around the critical resistive coating.

The invention will be described in greater detail in connection with the drawing in which the only figure is a resistor shown partly in cross-section to illustrate the inner parts thereof.

In the drawing a ceramic rod 11 having suitable electrical characteristics is coated with a resistive material 12, which may be of any type, such as the common carbonaceous or metallic materials, applied to the cylindrical surface of the rod by any convenient technique. At each end of the resistive material is a band of low-resistance material 13 and 14 to which electrical contact may be made rather easily without changing the resistance value of material 12. The resistive coating 12 may be grooved, as indicated by reference character 16, to increase the overall resistance and to make it easier to conrtol the resistance of the completed resistor.

At the ends of the rod 11 are two metal caps 17 and 18 to which individual wire terminals 19 and 21 are attached by a swaging process or by any other suitable method. In the drawing part of the resistor has been shown broken away to illustrate a pair of flanges 22 and 23 formed on the terminal 19 by the swaging process to grip the cap 17. Only the outer flange 25 is shown on the left-hand terminal 21.

The outer ceramic shell that encloses the resistive part 3,136,973 Patented June 9, 1964 of the overall structure is indicated by reference character 24 and is made of a material having substantially the same linear coefficient of thermal expansion as the rod 11. Metallized bands Mind 27 are formed on the inner surface of the shell 24 at each end thereof and extending far enough into the interior of the shell tooverlap the caps 17 and 18 when the resistive element is inserted into the shell. The shell 24 may be formed of the same ceramic material as the rod 11 while the metallized coatings 26 and 27 are normally made of a multiple layer consisting of a silver paste covered by a copper layer which, in turn, is covered by a tin coating to form a metallized band having a thickness of approximately one one-thousandth of an inch. The bands 26 and 27 extend far enough into the interior of the shell 24 so that the inner ends of these bands are approximately coplanar with the inner ends of the caps 17 and 18.

The shell 24 is slightly longer than the overall length of the ceramic rod 11 together with the caps 17 and 18 so that there is a space at each end of the shell 24. This a space is filled with molten solder which adheres to the outer surface of the cap and a small part of the wire lead attached thereto and the metallized band. The solder at the right-hand end of the resistor in the drawing is indicated by reference character 28 and that at the lefthand end is indicated by reference character 29. The solder also flows into the restricted, annular area between the cylindrical portions of the caps 17 and 18 and the bands 26 and 27. The solder in the annular regions, indicated by reference character 31 and 32, respectively, is between approximately one and six one-thousandths of an inch thick. y

The caps 17 and 18 are made of an alloy having approximately the same coeflicient of thermal expansion as the ceramic. In the case of an alumina ceramic having a coefficient of thermal expansion of approximately 5 10- units per unit length per degree centigrade, it has been found satisfactory to use end caps made of a nickel-iron alloy consisting of approximately 42% nickel and approximately 58% iron. The thickness of the caps is approximately five one-thousandths of an inch, which is of the same order of magnitude asthe thickness of the solder in the regions 31 and 32. With this arrangement it has been found that the metallized coating 26 and 27 does not pull away from the shell 24 in the region adjacent to the solder regions 31 and 32 nor does the shell itself crack at these points in spite of the fact that the solder has a coeflicient of thermal expansion of about 30O 1O units per unit length per degree centigrade. Thus, the regions 31 and 32 form impenetrable bands that prevent the atmosphere from reaching the sensitive resistive coating 12. 2

While this invention has been described in limited terms as it relates to a specific embodiment, it will be understood by those skilled in the art that the true scope is determined by the following claim and that modifications may therefore be made from the specific embodiment described without departing from the scope of the invention.

What is claimed is:

A resistor comprising a cylindrical ceramic rod; a coating of resistive material on the cylindrical surface of said rod; a pair of end caps on opposite ends of said rod electrically connected to opposite ends of said resistive material, each of said caps having a cylindrical portion extending a short distance along the cylindrical surface of said. rod and an end portion across the end of said rod, said caps being made of metal having a certain thickness and having substantially the same coefficient of thermal expansion as the ceramic material of said rod and being press-fitted on said rod; and an outer hermetic shell comprising a hollow ceramic tube having an inner diameter at least substantially equal to the outer diameter of the cylin- 3 4 drical portions of said caps; a pair of metallized bands on between the cylindrical portion of a respective one of the inner surface of said ceramic tube at the ends thereof, said caps and the adjacent metallized band.

each of said bands extending inwardly from the respective ends of said tube far enough to overlap the cylindrical portions of said end caps; and -a pair of solder plugs's'ealing the ends'of said tube, saidsolder plugs'being hardened 1,769,623 Chance et a1. July 1, 1930 inplgtcefroni moltenmetalg and each of saidplugs having 1,835,582 Allen Dec. 8, 1931 a tubular portion of substantially the same thickness as the 2,046,922 Minnium July 7, 1936 metal of said caps and extending into the annular region 2,597,338 Kohring May 20, 1952 

